How to make a JFrame Modal in Swing java

The only code that have worked for me:

childFrame.setAlwaysOnTop(true);

This code should be called on the main/parent frame before making the child/modal frame visible. Your child/modal frame should also have this code:

parentFrame.setFocusableWindowState(false);
this.mainFrame.setEnabled(false);

As others mentioned, you could use JDialog. If you don't have access to the parent frame or you want to freeze the hole application just pass null as a parent:

final JDialog frame = new JDialog((JFrame)null, frameTitle, true); frame.setModal(true);
frame.getContentPane().add(panel);
frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
frame.pack();
frame.setVisible(true);